1. Field of the Invention
The present invention relates in general to the art of power beaming and more particularly to a millimeter wave power beaming system.
2. Description of Related Art
Power beaming energy from a first location (e.g., on Earth, a high altitude suspended platform or a satellite) to a remote location (e.g., on Earth, a high altitude suspended platform or a satellite) has provided particular problems known only to those skilled in the art. Early attempts to develop a system for transmitting energy to power a remote device with a high degree of efficiency is described in, “The History of Power Transmission by Radio Waves” by William C. Brown, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-32, No. 9, September 1984, wherein the article describes research into an apparatus that allows power to be sent from the Earth to a device above the Earth via a 2.45 GHz radio signal.
Remote devices (e.g., high altitude platforms) if kept aloft by received transmitted energy, can be implemented in applications that include surveillance, weather and pollution monitoring, and communications. An altitude range between about 60,000 ft and about 100,000 ft is beneficial because it lays above the jet stream where the wind velocity is low and where the drag is minimum. At higher altitude, the reduction in atmospheric density makes flight difficult for these non-orbiting platforms. A benefit of utilizing such high altitude platforms is the stationary nature of the flight path, which gives a constant presence. Cost is also minimized when utilizing such platforms along with the ability to recover expensive payloads.
Platforms that carry their own fuel, both lighter than aircraft (balloons or airships) and conventional aircraft, cannot stay aloft for time periods (e.g., months to years) that are presently envisioned. Platforms that rely on solar power suffer from the weight of the large area solar cells and the requirement of energy storage for nighttime operation and higher than average peak propulsion loads.
An energy storage scheme proposed by NASA is to provide regenerative fuel cells whereby solar energy during the daytime is used to break water into hydrogen and oxygen. Fuel cells then recombine the two gases during the nighttime hours for the needed electrical power. However, such a scheme, while attractive in concept, provides for an efficiency of only about 50% and the weight is high, thereby lowering an allowed payload.
Power beaming using microwave frequencies has been discussed and partially demonstrated for several decades using low frequencies in the ISM band at 2.45 GHz. Such a long wavelength allows for inexpensive magnetron sources such as those used in microwave ovens that can be phase locked for large arrays. However, the size of the transmitting arrays is too large for mobile and tactical applications.
Conversion of such beam powered electromagnetic radiation into useful electrical power by a receiving antenna (i.e., a rectenna or rectifying antenna) has also been utilized in the past but has not received significant recent development. Such a device is typically mounted on the lower surface of an aircraft where it rectifies the electromagnetic energy of the appropriate wavelength into DC energy. The DC energy is then used to power the aircraft's propulsion system and on-board systems. Background information on beam powered systems that utilize a rectenna is described and claimed in U.S. Pat. No. 5,503,350, entitled “Microwave-Powered Aircraft,” issued Apr. 2, 1996 to Foot and in U.S. Pat. No. 5,068,669, entitled “Power Beaming System,” issued Nov. 26, 1991 to Koert et al.
The standard configuration for a rectenna is a pair of quarter wave dipole antennas connected by a diode to form the microwave analog of a half wave rectifier circuit. The combination of a quarter wave dipole and a diode form a half wave dipole antenna with current flowing in only one direction. While there have been attempts to fabricate a large area rectenna using such a configuration that includes a single Shottky diode as a rectifying element, several factors, such as low reverse breakdown voltages at the junction, low current densities during forward biasing, and high junction capacitances have combined to limit power densities on the rectenna, thus requiring large areas to convert the received millimeter wave energy to DC power. Moreover, the large rectenna panels add to the weight and cost of such a receiving system.
Background information for rectennas is described and claimed in U.S. Pat. No. 4,943,811, entitled “Dual Polarization Electromagnetic Power Reception And Conversion System,” issued Jul. 24, 1990 to Alden et al. Further information on rectennas is described in “A 35 GHz Extremely High Power Rectenna For The Microwave Lightcraft,” by Alden et al., American Institute of Physics, pp. 292-300, 2003. Additional information on rectennas is described and claimed in U.S. Pat. No. 5,043,739, entitled “High Frequency Rectenna,” issued Jan. 14, 1992 to Logan et al., and assigned to the assignee of the present application.
Accordingly, the present invention addresses a need for a system that includes a megawatt millimeter wave beam power source and a receiving system, such as an improved rectenna design, to enable a high altitude platform to operate at predetermined altitudes for extended periods of time over wide portions of the globe.